Power system for multiple power sources

ABSTRACT

A voltage booster allowing for increased utilization of low voltage, high current, unregulated DC power (“LVDC source”), such as, but not limited to, fuel cells, batteries, solar cells, wind turbines, and hydro-turbines. LVDC generation systems employing a variable low voltage DC-DC converter of the present disclosure may be used without a power inverter in applications requiring high voltage DC inputs and can also allow for the employment of common, low cost, reliable, low voltage energy storage chemistries (operating in the 12-48 VDC range) while continuing to employ the use of traditional inverters designed for high voltage power supplies. An embodiment of the DC boost converter includes a plurality of interleaved, isolated, full-bridge DC-DC converters arranged in a Delta-Wye configuration and a multi-leg bridge.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.15/209,707, filed Jul. 13, 2016, which is a continuation-in-part of U.S.patent application Ser. No. 14/194,773, filed Mar. 2, 2014, titled“Power Conversion System with a DC to DC Boost Converter,” and issued asU.S. Pat. No. 9,413,271, and claims the benefit of priority of U.S.Provisional Patent Application No. 61/781,965, filed Mar. 14, 2013, andtitled “Power Conversion System with a DC to DC Boost Converter”, eachof which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of powerelectronics. In particular, the present invention is directed to a PowerSystem for Multiple Power Sources.

BACKGROUND

There are many devices that either produce variable low voltage DC power(below 100 VDC) or require low voltage DC power. A common example of alow voltage DC power source is a fuel cell, which is an electrochemicaldevice which reacts hydrogen with oxygen to produce electricity andwater. The basic process is highly efficient, and fuel cells fueleddirectly by hydrogen are substantially pollution free. Yet, the outputof fuel cells (variable low voltage, high current DC power) makesefficient engineering solutions difficult, especially in residential andlight commercial applications, where the power output demands of a fuelcell are not as significant. While sophisticated balance-of-plantsystems are used for optimizing and maintaining relatively low powercapacity applications, they do not effectively meet residential andlight commercial power needs and at least a portion of this failure isattributable to the lack of effective power electronics to pair with lowvoltage DC power sources and loads.

For some low voltage DC sources such as fuel cells and batteries, thepower conversion efficiency degrades over time as the sources aredepleted. For example, for fuel cells, the fuel cell stack producesvariable low-voltage DC based on power demand and the stack's averagevoltage degrades over time based on catalyst loss and energy conversionefficiency degradation. Increasing the number of fuel cells running inparallel will increase the output stack voltage, but degradation willstill render the system inefficient and possibly unusable in arelatively short amount of time, requiring that the fuel cell stack bereplaced.

SUMMARY

In a first exemplary aspect, a power system for powering one or moreloads is described, the power system comprising: a plurality of powersources electronically coupled to the one or more loads and inelectronic communication with other ones of the plurality of powersources, and wherein at least one of the plurality of power sources isselected from the list of: a fuel cell stack, a battery, a capacitor, aflow battery, a solar panel, and a wind turbine; a plurality of powerconversion systems coupled to a corresponding respective one of theplurality of power sources and wherein each of said plurality of powerconversion system includes: a DC to DC boost converter (DDBC) forconverting power received from the power source, the DDBC having aplurality of interleaved, isolated, full-bridge DC-DC convertersarranged in a Delta-Wye configuration and a multi-leg bridge.

In another exemplary aspect, a power system for powering one or moreloads is described, the power system comprising: a plurality of powersources electronically coupled to the one or more loads and inelectronic communication with other ones of the plurality of powersources, and wherein at least one of the plurality of power sources is afuel cell stack and wherein at least one other one of the plurality ofpower sources is selected from the list of: a battery, a capacitor, asolar panel, and a wind turbine; a plurality of power conversion systemscoupled to a corresponding respective one of the plurality of powersources and wherein each of the plurality of power conversion systemincludes: a DC to DC boost converter (DDBC) for converting powerreceived from the power source, the DDBC having a plurality ofinterleaved, isolated, full-bridge DC-DC converters arranged in aDelta-Wye configuration and a multi-leg bridge; and a system controllerconfigured to monitor the total system load required by the one or moreloads and to apportion the total system load across the individual powersources by communicating with and coordinating the other power controlsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show aspectsof one or more embodiments of the invention. However, it should beunderstood that the present invention is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a graph of commercial inverter efficiency versus loadpercentage for DC sources;

FIG. 2 is a block diagram of a power conversion system according to anembodiment of the present invention;

FIG. 3 is a block diagram of another power conversion system accordingto an embodiment of the present invention;

FIG. 4 is a block diagram of another power conversion system accordingto an embodiment of the present invention;

FIG. 5 is an electrical schematic of a DC to DC (DC-DC) boost converteraccording to an embodiment of the present invention;

FIG. 6 is a process diagram of providing high voltage DC power to a loadfrom a low voltage DC power source;

FIG. 7 is a block diagram of a computing system suitable for use with aDC-DC boost converter according to embodiments of the present invention;

FIG. 8 is a block diagram of a power system according to an embodimentof the present invention; and

FIG. 9 is a process diagram of a control strategy suitable for use witha power system according to an embodiment of the present invention.

DESCRIPTION OF THE DISCLOSURE

There are many DC power sources that produce low voltage, high current,unregulated DC power (“LVDC source”). However, many of these LVDCsources are relatively incompatible with commercially availableinverters insofar as their efficiencies and usefulness are impeded bythe high input voltage required for efficient operation of these typesof inverters. As an example of this phenomenon, FIG. 1 shows a graph 10of inverter efficiency versus load percentage for a solar power source,i.e., a high voltage source, and a LVDC source, where a traditionalinverter is used, i.e., one typically designed to receive high voltageinputs. As shown, the power conversion efficiency of power coming fromthe solar source varies as a function of load and is represented bysolar voltage input 14. Solar voltage input 14 reaches a maximum, inthis example, of about 93% conversion at between about 50 and 75 percentof maximum load. In comparison, the power conversion efficiency of theLVDC source at its beginning of life (BOL) (represented by BOL voltageinput 18) is considerably less. At its peak, the LVDC source obtains anefficiency of about 85% at between about 65 and 75 percent of maximumload. Moreover, as the LVDC source ages, there is an even greaterdisparity in power conversion when compared to the solar source. At theend of life (EOL) (represented by EOL voltage input 22) of LVDC sources,the power conversion efficiency peaks at about 70% at 70% maximum load.

The differences between the voltage inputs are designated as loss areas26, i.e., loss area 26A and B, and represent inefficiencies inconversion of power. Loss area 26A is the difference between solarvoltage input 14 and BOL voltage input 18 and loss area 26B is thedifference between solar voltage input 14 and EOL voltage input 22. Lossarea 26B is greater than loss area 26A because while solar voltage input14 may decrease over time, the longevity of the solar source farsurpasses that of LVDC power sources, such as batteries and fuel cells.The difference between BOL voltage input 18 and EOL voltage input 22 islargely due to the decreased ability for the LVDC source to generatesufficiently high output voltages to reduce inefficiencies related tothe use of inverters designed for high voltage power sources.

A variable low voltage DC-DC converter according to present disclosureallows for improved and increased use of numerous power sources, suchas, but not limited to, fuel cells, batteries, solar cells, flowbatteries, wind turbines, and hydro-turbines. The variable low voltageDC-DC converter allows for optimization of a distributed generationsystem including a LVDC source and an energy storage device forefficiency, life expectancy and cost without being limited to highvoltage outputs from the LVDC source. LVDC generation systems employinga variable low voltage DC-DC converter of the present disclosure may beused without a power inverter in applications requiring high voltage DCinputs, such as a vehicle or other battery charger, a beater, a welder,a motor starter, a motor, a high voltage DC (HVDC) utility application,a telecommunications equipment, a vehicle, a tractor, a marine auxiliarypower, and a material handling equipment. A variable low voltage,bi-directional DC-DC converter according to the present disclosure canalso allow for the employment of common, low cost, reliable, low voltageenergy storage chemistries (operating in the 12-48 VDC range) whilecontinuing to employ the use of traditional inverters designed for highvoltage power supplies. The variable low voltage DC-DC converter alsocan simplify the design (by reducing components required) and increasethe useful life of the LVDC sources while allowing for efficientcharging and discharging to a high voltage DC application.

Additionally, incorporating LVDC power storage with highly efficientgenerating sources such as fuel cells, solar or wind increases theeconomic viability of the generating device by capturing more useableenergy and allowing for the use of low cost LVDC power storage employingbatteries technologies, such as lead acid batteries. For example,vehicle and residential energy storage systems are dominated by batterychemistries allowing for high voltage storage topologies. Lithium-ionand nickel-metal-hydride are two examples of batteries that operate inthe 400-600 VDC range so as to ensure high voltage DC output tocommercially available inverters for high efficiency conversion. Highvoltage storage topologies are expensive and can only accept power atcertain voltages. Use of a variable low voltage DC-DC converter allowsfor use of produced power at lower voltages to be stored within thebatteries as well as the use of lower cost batteries, such as lead acid.

Referring now to FIG. 2, a high-level block diagram of a powerconversion system 100 is shown that converts a variable low DC voltagefrom a DC source 104 into an AC voltage for an AC load 108. Examples ofa DC source 104 can include a variety of variable DC power sources thatare typically subject to low voltage cutoffs, such as, but not limitedto, a solar array, a wind turbine, a fuel cell, a flow battery, a waterturbine, a battery, and a capacitor. Examples of AC load 108 include,but are not limited to, an electric power grid, a vehicle, and aresidence.

As shown in FIG. 2, power conversion system 100 includes a DC-DC boostconverter (DDBC) 120, an inverter 124, and a control system 128. Asdescribed in more detail below with reference to FIG. 3, DDBC 120converts variable, low voltage DC to constant, high voltage DC using thehigh current output of the DC source 104. Once converted to fixed, highvoltage DC, the voltage is delivered to inverter 124, which converts theDC voltage to an AC voltage suitable for distribution to AC load 108.The operation of DDBC 120 is regulated by control system 128 that, at ahigh level controls, among other things, the input current level and aset-point for inverter 124 so as to draw power from the DC source.Control system 128 is described in more detail below with respect toFIG. 4.

FIG. 3 shows an alternative embodiment of power conversion system 200employing a DDBC 120. In this embodiment, power conversion system 200 isdesigned and configured so as to receive power from both a DC source,i.e., DC source 104, and an AC power source 208 (e.g., the electricalgrid). DDBC 120 receives inputs from DC source 104 and AC power source208 (via inverter 212) and converts the inputs to a low, high current,DC voltage for a low DC voltage storage/load 204. Examples of low DCvoltage storage/load 208 include, but are not limited to, an electricvehicle, a material handling equipment (e.g., electric fork-lifts), atractor, a marine auxiliary power system, and a telecommunicationsequipment.

As is well known, a DC to DC converter is a species of power converter.Moreover, many power converters can be configured to convert DC into AC(typically called inverters), as well as many other power conversionfunctions. Thus, DDBC 120 can, in some embodiments, be implemented by amulti-function power converter that is configured to operate as aconverter, i.e., convert DC into DC, or inverter, i.e. to convert DCinto AC, or rectifier, i.e. to convert AC into DC. However, in someembodiments, DDBC 120 is implemented by a device that can only convertDC into DC, DC into AC or AC into DC.

Turning now to FIG. 4, there is shown a more detailed embodiment ofpower conversion system 100. In this embodiment, power conversion system100 includes input terminals 132, allowing mechanical and electricalconnection to DC source 104 and the other components of the powerconversion system. At the point of power input to DDBC 120 is a sensor136, which measures an input current 140 coming from DC source 104 andtransmits a signal 144, representative of the input current, to controlsystem 128.

Input current 140 is then fed into an input current filter 148. In anexemplary embodiment, input current filter 148 is an inductor/capacitorcombination that filters input current 140 to protect DC source 104 fromthe effects of AC ripple caused by switching of transistors, e.g.,MOSFETS (high frequency), and feedback from the grid (low frequency).This AC ripple can cause localized voltage changes in electrodes, whichresults in increased fuel consumption and loss of efficiency in fuelcells, Faradaic heating in batteries and disruptions in peak powertracking algorithms, thereby leading to loss of efficiency indistributed generation devices.

In an exemplary embodiment, when DDBC 120 is configured as a pulse widthmodulated (PWM) boost converter, the DDBC receives the filtered currentinput from input current filter 148 and converts the low voltage DC tohigh voltage DC in three steps. Power conversion system 100 converts thelow voltage DC signal to a low voltage AC signal, then converts the lowvoltage AC signal to high voltage AC signal, then converts the highvoltage AC signal to a high voltage DC signal. Two, interacting controlloops are used in conjunction with DDBC 120.

For high-power applications, increasing the switching frequency of thetransistors (as a part of, for example, MOSFET-based DC-AC converters304 discussed in more detail below) provides the benefit of reducing thesize of associated passive devices included with a DDBC, such as DDBC120, such as transformers, inductors and capacitors. Size reductions inthe aforementioned components beneficially reduce the overall footprintof the device and the cost. However, higher switching frequencies leadto higher switching losses and lower conversion efficiencies. In anexemplary embodiment, of DDBC 120, DDBC 120 controls the switchingfrequency of the transistors based on operating conditions, therebyallowing the size of passive devices to be minimized while mitigatingthe effects of switching losses. For example, DDBC 120 can be designedwith components sized for maximum power output at maximum switchingfrequency where the effects of maximum switching loss are minimized, butcan also reduce the switching frequency when power demands are lower.This configuration of DDBC 120 can be useful in distributed generationapplications where the demand on the power source could remain low,e.g., <20% rated capacity, for extended periods of time as withoutmodification of the switching frequency, switching losses would have anadverse effect on power system efficiency. By reducing the switchingfrequency as the load on the power sources decreases, switching lossesat low loads can be lessened. In an exemplary embodiment, DDBC 120 isconfigured such that the drop in switching frequency is proportional tothe load decrease monitored by the system.

Regulation of the voltage boost provided by DDBC 120 can be accomplishedusing one or more feedback or control loops. For example, and as shownin FIG. 4, a feedback loop 152 is electronically coupled to DDBC 120 andcontrol system 128. Feedback loop 152, is an inner, fast loop of powerconversion system 100, which regulates the voltage boost of thetransformers by receiving, as an input, a fixed input voltage signal,representative of the voltage on inverter 124, and adjusting the phaseangles of the voltages of the power sent from the DC source on thetransformer primary windings (discussed and shown in FIG. 5) by using,for example, phase-shifting modulation technique whose algorithms residein the microprocessor. The DC source voltage will vary based on severalvariables including: output current, level of degradation, and controlmethods such as peak power tracking (PPT) employed by the controlsystem. In one embodiment, feedback loop 152 transmits power at areference voltage from a power supply (not shown) to the transformerprimary windings to adjust the voltage phase angles seen by thetransformers; the higher the applied voltage, the lower the duty ratioof the transformers and the lower the voltage boost and vice versa: thelower the applied voltage, the higher the duty ratio and the higher theboost. In another embodiment, feedback loop 152 receives referencevoltage power from a dedicated circuit (not shown) that draws power fromDC source 104 and fixes its voltage through an additional dedicatedtransformer arrangement designed especially for this task, such as onewith primary windings connected in the delta configuration and secondarywindings connected in wye to produce a desired phase-shift.

An output current sensor 156 is coupled between DDBC 120 and controlsystem 128. Output current sensor 156 measures the current exiting DDBC120 and sends a signal 160 representative of the current to controlsystem 128.

The high DC voltage power generated by DDBC 120 is transmitted toinverter 124 (through an output current filter, such as output filter316 shown in FIG. 5) for conversion into high voltage AC suitable forintroduction to the AC load 108. The amount of power available to ACload 108 is dependent upon, among other things, the available outputcurrent of DC source 104. Control over the power draw to AC load 108 canbe accomplished using control loop 154, which is electrically coupled tocontrol system 128 and output current sensor 156. In an exemplaryembodiment, control loop 154 provides a power draw command to inverter124 based upon the available output current of DC source 104 (determinedfrom a signal received from output current sensor 156). If DC source 104cannot meet the power draw command, control system 128 de-rates powerconversion system 100. In another exemplary embodiment, control system128 can sense power demands of AC load 108 (via voltage drop or otherindication not shown) and responsively increase demand on DC source 104.In this embodiment, control loop 154 monitors output current sensor 156to ensure voltage demand is met using DC link capacitors to maintainvoltage at a level set by the microprocessor. If the voltage level isnot reached, control loop 154 signals to control system 128 that powerconversion system 100 needs to be derated.

FIG. 5 shows an embodiment of a DC-DC boost converter, DDBC 300,suitable for use in the power conversion systems described herein. Thetopology of DDBC 300 can be described as three interleaved, isolated,full-bridge DC-DC converters in a Delta-Wye configuration followed by asynchronous, three-leg bridge. In this embodiment, three phases areused. However, more or fewer phases may be implemented. Splitting theconversion into three phases allows the high current to be spread acrossmultiple legs thereby increasing efficiency and heat dissipation. Thefigure shows, first, a plurality of MOSFET-based DC-AC converters 304,e.g., 304A-C, that are electrically coupled to an input current filter,such as input current filter 148.

A plurality of transformers 308, e.g., transformers 308A-C, areelectrically coupled to the DC-AC converters, such that each transformeris coupled to one or more of the DC-AC converters. Thus, for example,transformer 308A is coupled to DC-AC converter 304A and 304B.Transformers 308 convert the low voltage AC generated by converters to arelatively higher voltage AC. Typically, the number of transformers 308used in DDBC 300 corresponds to the number of converters 304. The amountof boost provided by DDBC 300 is regulated by the turns ratio of thetransformers 308: the higher the turns ratio, the higher the boost. Anundesirable side effect of increasing the turns ratio of thetransformers is higher flux losses. However, by increasing the number oflegs or phases, the duty on each transformer 308 is reduced, and theturns ratio and the flux losses are both lower. Moreover, byinterleaving, or summing, the legs after transformers 308, DDBC 300achieves a higher overall effective boost with minimal flux losses.Also, since the MOSFET-based topology is inherently high switchingfrequency, the high-frequency transformers 308 have the advantages ofbeing small in size and highly efficient.

Additionally, in grid-tied applications, transformers 308 fulfill therequirement of grid isolation. Isolation requirements take two forms:power and signal. Power isolation is the minimization of DC currentinjected into the grid and is a stringent requirement of most utilities.Transformerless inverters are being introduced in Europe, which whilesmaller and lighter than their transformer-based counterparts, thesedevices suffer from lower efficiencies and additional components toprevent DC leakage onto the grid. Signal isolation is the separation ofgeneration-side measurements from the grid to ensure critical parametermeasurement accuracy. The integration of small, highly efficienthigh-frequency transformers, such as transformer 308, fulfills allisolation requirements with a topology solution whose complexity andcost is less than a transformerless topology.

The high voltage AC output by transformers 308 is fed to a correspondingnumber of synchronous rectifiers 312, e.g., 312A-C. Synchronousrectifiers 312 convert the high voltage AC from transformers 308 to highvoltage DC. In an exemplary embodiment, synchronous rectifiers 312 areMOSFET-based rectifying devices. Typically, the number of synchronousrectifiers 312 included with DDBC 300 is the same as the number oftransformers; however, more or fewer may be used. As shown in FIG. 5,three MOSFET-based synchronous rectifiers are electrically coupled totransformers 308. Before the output of DDBC 300 is provided to aninverter, such as inverter 124. it can pass through an output filter316. Output filter 316 protects the power conversion system from ACripple from the electrical grid (if so coupled). AC ripple from theelectrical grid will affect the performance of the power conversionsystem inductors, capacitors, switches and the low voltage DC source,e.g., fuel cell.

Overall, a power conversion system including DDCB 300 is based on aboost topology which features continuous output current, a voltage ratioless than one, and discontinuous input current. AC-DC converters 304 areoperated in parallel to help distribute dissipated heat, lower theswitch RMS current, and enable soft switching features. AC-DC converters304 are also interleaved in such fashion that when one phase stopsconducting current and its phase transformer freewheels, another phasestarts or already is conducting current. Current is thereby spreadacross all phases. Input current is always pulsed. A noted benefit ofthis topology is that power conversion efficiency increases as the inputvoltage decreases. This is counter to most power electronics behaviorwhich is just the opposite, conversion efficiency increases as voltageincreases due to a reduction in resistive losses. However, the topologydescribed herein requires higher performance, and therefore an increasedduty ratio on the switching devices, leading to reduced switching lossesthat outweigh an increase in resistive losses due to lower inputvoltage.

For DDBC 300, the input currents of all three phases sum, thus,depending on the operational mode the total input current could be foundeither between zero and one phase current (which is n×the outputcurrent) or between one and two phase currents; hence, the total inputcurrent resembles that of a multilevel converter. The system willinclude at least two control loops, as described above, which willregulate both the output current and output voltage.

Benefits of the average current control method are infinite DC gain,better control of inductor current, and immunity to noise. A lack ofinherent peak current capability is a disadvantage of this controlmethod, along with a limited current loop bandwidth characteristic tolinear control systems.

Power conversion system 100 as described herein greatly extends therange over which a DC source, such as DC source 104, can operate. Forexample, using power conversion system 100 enables a fuel cell system tocontinue to provide power to a utility grid closer as the output voltageof the fuel cell system decreases over time.

Control system 128 is designed and configured to manage the componentsof power conversion systems 100 (FIGS. 2 and 4) or 200 (FIG. 3) bycollecting information from inputs internal and external to the system,such as, but not limited to sensed input voltage, sensed input current,output voltage set point, and heat sink temperature. Informationcollected by control system 128 is input into programmed algorithms, setpoints, or lookup tables so as to determine operating parameters forpower conversion system 100, components, such as DDCB 120, controlsignals, feedback loops and/or to generate external data for use inevaluating the efficiency, lifespan, or diagnosing problems with thepower conversion system. Although control system 128 is presentlydescribed as a separate component of power conversion system 100, it isunderstood that control system 128 can be dispersed among the variouscomponents described herein without affecting the function of the powerconversion system.

Turning now to an exemplary operation 400 of a power conversion system,such as power conversion system 100, and with reference to exemplaryembodiments shown in FIGS. 1-4 and in addition with reference to FIG. 6.

At step 404, measure the output voltage of the DC source, such as DCsource 104. Measurement of the output voltage may be via a sensor, suchas sensor 136, which is capable of transmitting a signal representativeof the voltage to a control system, such as control system 128.

At step 408, the measurement from step 404 is compared to priormeasurement of the output voltage, an initial voltage output value (atinitial commissioning, the DC source, such as a fuel cell stack, willhave its highest beginning of life (BOL) voltage curve for poweroutput), a maximum power output value (as determined in step 420discussed below) or a predetermined set-point or threshold value. If theoutput voltage measured in step 404 has decreased below a certainpercentage (when compared to a prior measurement or the initial voltageoutput value) or has fallen below the predetermined set-point orthreshold value, method 400 proceeds to step 412; otherwise, the methodproceeds to step 416.

At step 412, a determination is made as to whether the DC source canincrease its current output so as to meet the rated power output of theDC source. If the DC source can increase its current output, the methodproceeds to step 416; if not, the method proceeds to step 420.

At step 416, current is drawn from a DC source into the power conversionsystem at a value sufficient to supply the inverter, such as inverter124, with sufficiently high DC voltage to efficiently operate theinverter. Process 400 then returns to step 404 to remeasure the outputvoltage of the DC source. Notably, use of a power conversion system,such as power conversion system 100, and especially DDBC's describedherein, can allow for a significant drop in DC source output voltage (incomparison to an initial voltage output value) and still providesufficient power to the inverter. In an exemplary embodiment, thevoltage drop from a commissioned value of the DC power source can be asmuch as 80%.

At step 420, if sufficient current cannot be drawn from the DC source toeffectively operate the inverter, it is determined whether or not thepower conversion system can be derated such that the maximum poweroutput of the system is based on maximum voltage sustainable by the DCsource. Sufficient current to maintain the voltage may be unavailablebecause other balance-of-plant-components of the DC source, such as thestack air supply blower or the waste heat rejection loop of a fuel cellpower system, limit the output of the DC source. If the power conversionsystem cannot be derated, the system is removed from service. Otherwise,the power conversion system is derated at step 424 such that a maximumpower output of the DC source is transmitted to step 408 for furthercomparisons between the actual output voltage measured in step 404.

At step 428, a determination is made as to whether or not the DC sourcecan continue to supply power to the inverter in an amount suitable toefficiently run the inverter. If not, the DC source is taken out ofservice at step 432 and the DC source is replaced or recharged. If theDC source can provide a suitable power output to efficiently run theinverter, process 400 returns to step 404.

A power conversion system as described herein allows for: optimizationof a fuel cell, energy storage, and distributed generation system forefficiency, life and cost instead of voltage output; recovery, useand/or storage of additional power from these devices that until nowwould be unobtainable; increasing the useable life of the DC sourceportion of the system, e.g., a fuel cell stack, a battery string, asolar panel, a flow battery, a wind turbine or a water turbine; allowingDC sources to be used without an inverter stage in applicationsrequiring high voltage DC inputs, e.g., a vehicle or other batterycharger, a heater, a welder, a motor starter, a motor, a high voltage DC(HVDC) utility application, telecommunications equipment, a vehicle, atractor and/or a marine auxiliary power, a material handling equipment;or providing additional power and life to improve the economics of thedistributed power generation systems. Additionally, the bi-directionalcapability of the device allows for the implementation of low voltagebattery storage, charged by relatively high power sources allowing forthe employment of readily available, low cost, reliable energy storagesystems such as lead acid battery.

FIG. 7 shows a diagrammatic representation of one implementation of amachine/computing device 500 that can be used to implement a set ofinstructions for causing one or more control systems of power conversionsystem 100, for example, control system 128, to perform any one or moreof the aspects and/or methodologies of the present disclosure. Device500 includes a processor 504 and a memory 508 that communicate with eachother, and with other components, such as DDBC 120 and inverter 124, viaa bus 512. Bus 512 may include any of several types of communicationstructures including, but not limited to, a memory bus, a memorycontroller, a peripheral bus, a local bus, and any combinations thereof,using any of a variety of architectures.

Memory 508 may include various components (e.g., machine-readable media)including, but not limited to, a random access memory component (e.g, astatic RAM “SRAM”, a dynamic RAM “DRAM”, etc.), a read-only componentand any combinations thereof. In one example, a basic input/outputsystem 516 (BIOS), including basic routines that help to transferinformation between elements within device 500, such as during start-up,may be stored in memory 508. Memory 508 may also include (e.g., storedon one or more machine-readable media) instructions (e.g., software) 520embodying any one or more of the aspects and/or methodologies of thepresent disclosure. In another example, memory 508 may further includeany number of program modules including, but not limited to, anoperating system, one or more application programs, other programmodules, program data, and any combinations thereof.

Device 500 may also include a storage device 524. Examples of a storagedevice (e.g., storage device 524) include, but are not limited to, ahard disk drive for reading from and/or writing to a hard disk, amagnetic disk drive for reading from and/or writing to a removablemagnetic disk, an optical disk drive for reading from and/or writing toan optical media (e.g., a CD, a DVD), a solid-state memory device andany combinations thereof. Storage device 524 may be connected to bus 512by an appropriate interface (not shown). Example interfaces include, butare not limited to, SCSI, advanced technology attachment (ATA), serialATA, universal serial bus (USB), IEEE 1395 (FIREWIRE), and anycombinations thereof. In one example, storage device 524 may beremovably interfaced with device 500 (e.g., via an external portconnector (not shown)). Particularly, storage device 524 and anassociated machine-readable medium 528 may provide nonvolatile and/orvolatile storage of machine-readable instructions, data structures,program modules, and/or other data for control system 128. In oneexample, instructions 520 may reside, completely or partially, withinmachine-readable medium 528. In another example, instructions 520 mayreside, completely or partially, within processor 504.

Device 500 may also include a connection to one or more sensors, such assensor 136 and/or output current sensor 156. Sensors may be interfacedto bus 512 via any of a variety of interfaces (not shown) including, butnot limited to, a serial interface, a parallel interface, a game port, aUSB interface, a FIREWIRE interface, a direct connection to bus 512, andany combinations thereof. Alternatively, in one example, a user ofdevice 500 may enter commands and/or other information into device 500via an input device (not shown). Examples of an input device include,but are not limited to, an alpha-numeric input device (e.g., akeyboard), a pointing device, a joystick, a gamepad, an audio inputdevice (e.g., a microphone, a voice response system), a cursor controldevice (e.g., a mouse), a touchpad, an optical scanner, a video capturedevice (e.g., a still camera, a video camera), touchscreen, and anycombinations thereof.

A user may also input commands and/or other information to device 500via storage device 524 (e.g., a removable disk drive, a flash drive)and/or a network interface device 536. A network interface device, suchas network interface device 536, may be utilized for connecting device500 to one or more of a variety of networks, such as network 540, andone or more remote devices 544 connected thereto. Examples of a networkinterface device include, but are not limited to, a network interfacecard, a modem, and any combination thereof. Examples of a networkinclude, but are not limited to, a wide area network (e.g., theInternet, an enterprise network), a local area network (e.g., a networkassociated with an office, a building, a campus or other relativelysmall geographic space), a telephone network, a direct connectionbetween two computing devices and any combinations thereof. A network,such as network 540, may employ a wired and/or a wireless mode ofcommunication. In general, any network topology may be used. Information(e.g., data, instructions 520, etc.) may be communicated to and/or fromdevice 500 via network interface device 544.

Device 500 may further include a video display adapter 548 forcommunicating a displayable image to a display device 522. Examples of adisplay device 522 include, but are not limited to, a liquid crystaldisplay (LCD), a cathode ray tube (CRT), a plasma display, and anycombinations thereof.

In addition to display device 522, device 500 may include a connectionto one or more other peripheral output devices including, but notlimited to, an audio speaker, a printer and any combinations thereof.Peripheral output devices may be connected to bus 512 via a peripheralinterface 556. Examples of a peripheral interface include, but are notlimited to, a serial port, a USB connection, a FIREWIRE connection, aparallel connection, a wireless connection, and any combinationsthereof.

A digitizer (not shown) and an accompanying pen/stylus, if needed, maybe included in order to digitally capture freehand input. A pendigitizer may be separately configured or coextensive with a displayarea of display device 552. Accordingly, a digitizer may be integratedwith display device 552, or may exist as a separate device overlaying orotherwise appended to display device 552.

Turning now to a discussion of FIG. 8, there is shown an exemplary powergeneration and storage system 600 (hereinafter referred to as “powersystem 600”) according to an embodiment of the present disclosure. Powersystem 600 includes a plurality of independent power sources 604, whichcan be, but are not limited to, fuel cells, flow batteries, solarpanels, batteries, capacitors, and wind turbines. Coupled to each ofthese power sources 604 (or groups of power sources, such as a solarpanel array or a plurality of wind turbines) is a power conversionsystem, such as power conversion system 100, which is then coupled to abus 608 for transmission of electrical power to a load or the receptionof electrical power. For example, and as shown in FIG. 8, three fuelcells (power sources 604A-604C), two battery banks (power sources 604Dand 604E), and one solar array (power source 604F) are coupled to powerconversion systems 100A-100F, respectively. Each power conversion system100A is then coupled to bus 608. As described in more detail below, whenthere is a demand for power, one or more of the power sources 604 areemployed to deliver power to the load and, concomitantly or after,provide recharge power to the battery banks.

In an exemplary embodiment, power system 600 is designed and configuredto maximize fuel cell stack/module life, maximize overall systemefficiency, and minimize operating expense of the power system. Powersystem 600 achieves these outcomes by employing the exemplary controlstrategy 700 shown in FIG. 9.

At a high level, control strategy 700 dynamically operates and assignsload contributions from a power system, such as power system 600, basedon individual stack/module condition, performance, cumulative runtimeand number of stack/module start/stop sequences.

At step 704, a system controller is designated from amongst a pluralityof power control systems, such as power control system 100, in use bythe power system. In an exemplary embodiment, each power source or acollection of power sources includes a power control system with one ofthose power control systems or an additional power control system beingdesigned a system control.

At step 708, the system control monitors total system load. In anexemplary embodiment, total system load can be determined by monitoringthe current required by the load that is coupled to a power system, suchas power system 600.

At step 712, the system control apportions the total system load acrossthe individual power sources by communicating with and coordinating theother power control systems (hereinafter, referred to as, “secondarycontrollers”). Depending on the monitored total system load, the systemcontrol may also control whether and which power sources are switchedin/out from the bus. Moreover, the system control may control thesequencing of the power sources so that different ones of the powersources are switched in/out of operation. Control over sequencing can bevaluable when dealing with a power source such as a battery or fuelcells, whose useful life is impacted by use and extent of use.

Control strategy 700 assists in maximizing power source life, especiallywhen the power source is a fuel cell stack/module, maximizing overallsystem efficiency, and minimizing total operating expense of the powersource. For example, control strategy 700 can operate a plurality ofpower sources, such as a group of fuel cells, by for example, operatingthe fuel cells according to the load and, preferably (and if possiblegiven the load), avoiding operating the fuel cells inefficiency. Ifother power sources are available, e.g., a battery, control strategy 700may use the power available in the battery to avoid running the fuelcells inefficiently.

In an exemplary embodiment, a control strategy, such as control strategy700, when used in conjunction with a power system, such as power system600, can control the prioritization and load contribution of other powersource/storage assets (batteries, ultra-capacitors, solar, wind, etc.).Prioritization can extend the useful life of power sources and ensurethat power sources are being used efficiently.

Exemplary embodiments have been disclosed above and illustrated in theaccompanying drawings. It will be understood by those skilled in the artthat various changes, omissions, and additions may be made to that whichis specifically disclosed herein without departing from the spirit andscope of the present invention.

What is claimed is:
 1. A power conversion system comprising: a pluralityof interleaved, isolated, full-bridge DC-DC converters arranged in aDelta-Wye configuration; a control system, the control system designedand configured to de-rate the power conversion system to a lower powerrating, wherein the control system includes a control loop and afeedback loop. wherein the control loop monitors a continuous currentoutput and a discontinuous input current and determines a power drawcommand based upon the continuous current output and the discontinuousinput current, and wherein the feedback loop regulates the continuouscurrent output; and a synchronous, multi-leg bridge, wherein the powerconversion system is configured to receive power from a variable, lowvoltage DC source and convert the power to a more constant, highervoltage DC power.
 2. The power conversion system according to claim 1further including a plurality of DC-AC converters.
 3. The powerconversion system according to claim 2 further including a plurality oftransformers, wherein each of said plurality of transformers iselectronically coupled to a corresponding respective one of saidplurality of DC-AC converters.
 4. The power conversion system accordingto claim 3, wherein each of the plurality of transformers has a turnsratio of greater than 1:5.
 5. The power conversion system according toclaim 3, wherein the power conversion system is electronically coupledto an electrical grid and the plurality of transformers are designed andconfigured to provide isolation from the electrical grid.
 6. The powerconversion system according to claim 3, wherein the power conversionsystem includes a plurality of synchronous buck converters, eachelectrically coupled to a corresponding respective one of the pluralityof transformers.
 7. The power conversion system according to claim 6,wherein a combined output of the plurality of synchronous buckconverters is a continuous output current and a voltage ratio of lessthan one.
 8. The power conversion system according to claim 1, whereinthe power conversion system has a boost ratio of greater than 5:1. 9.The power conversion system according to claim 1, wherein the powerconversion system is bi-directional.
 10. The power conversion systemaccording to claim 1, wherein conversion efficiency increases as inputvoltage from the low voltage DC source decreases.
 11. The powerconversion system according to claim 1, wherein the power conversionsystem has a configurable boost ratio that varies with a DC-voltageoutput set point and a DC-voltage input from the low voltage DC source.12. A method for converting power from a low voltage DC source,comprising: receiving a low voltage DC signal through an input currentfilter; splitting the low voltage DC signal into a plurality of lowvoltage DC signals; converting each of the plurality of low voltage DCsignals to corresponding ones of a plurality of low voltage AC signals:converting each of the plurality of low voltage AC signals tocorresponding ones of a plurality of higher voltage AC signals; summingthe plurality of higher voltage AC signals to form a high voltage ACsignal; and converting the high voltage AC signal to a high voltage DCsignal.
 13. The method for converting power according to claim 12,further including outputting the high voltage DC signal to an electricalgrid.
 14. A power conversion system for converting a variable, lowvoltage DC signal to a more constant, higher voltage DC signal, thepower conversion system comprising: an input current filter; a pluralityof DC-AC converters electrically coupled to the input current filter; aplurality of transformers, each of the plurality of transformerselectrically coupled to a corresponding respective one of each of theplurality of DC-AC converters: a plurality of synchronous rectifiers,each of the plurality of synchronous rectifiers electrically coupled toa corresponding respective one of the plurality of transformers; acontrol system, the control system designed and configured to de-ratethe power conversion system to a lower power rating, wherein the controlsystem includes a control loop and a feedback loop, wherein the controlloop monitors a continuous current output and a discontinuous inputcurrent and determines a power draw command based upon the continuouscurrent output and the discontinuous input current, and wherein thefeedback loop regulates the continuous current output; and an outputfilter, wherein the plurality of DC-AC converters, the plurality oftransformers, and the plurality of synchronous rectifiers are configuredto form a plurality of interleaved, full-bridge DC-DC boost converters.15. The power conversion system according to claim 14, wherein each ofthe plurality of transformers has a turns ratio of greater than 1:5. 16.The power conversion system according to claim 14, wherein the pluralityof DC-DC boost converters collectively have a boost ratio of greaterthan 5:1.